A composite is a structure having multiple layers of woven or unwoven fiber or weave impregnated with a polymer material such as epoxy. Composites are tailorable so that by careful selection of material for the fiber layers, the number of layers and orientation of the layers, a composite may be fabricated having a selected set of characteristics. Composites are capable of providing, for example, strength characteristics comparable to those of metals with significantly less weight and with greater corrosion resistance. These materials are therefore frequently employed in aerospace applications and in other application where material failure can have catastrophic consequences. However, in order for these materials to perform to their design capabilities, there must be no breaks or other flaws in any of the fiber layers, either as originally fabricated or in use. Consequently, acceptance criteria for these structures are quite stringent.
However, detection of flaws in modern composites, which may for example have eight to thirty-two or more fiber layers, is a difficult task, particularly since the composites are generally anisotropic (i.e., have properties that differ depending on the direction of measurement).
If parts having anomalies/defects could be found and discarded early in the manufacturing process, tremendous savings in processing and labor would result. Also, if better post-production detection techniques were available, current large margins of error for composite components could be cut, allowing less expensive, more weight efficient structures, thereby permitting the full benefits of composites to be realized.
Current inspection techniques for such flaws/anomalies include visual, ultrasound and x-ray examination. Visual examination detects any surface breaking cracks; however, internal defects cannot be discovered by visual inspection. Ultrasound techniques locate areas of internal disbonding which are parallel to the surface, such as ply separation or failure to bond. Conventional x-ray technology can in principle reveal fiber breaks and irregularities in the fiber weave. However, normal x-ray shadowgraph technique may be overwhelmed by the superposition of many different layers in complex composites.
None of these techniques can identify the specific layer containing fiber defects, a critical parameter in determining the ultimate strength of a part. Since conventional x-ray techniques produce an image showing superposition of the many different layers, the overlay image of even eight separate layers of material, each having its own characteristic weave pattern, is extremely difficult to deconvolve visually. As can be seen in FIG. 1(a), which illustrates an image of a multiple layer test specimen obtained using the best current x-ray inspection technique, discontinuities are not obvious, and the layer containing the discontinuities cannot be identified. Inspection is often difficult and painstaking. Since the weave pattern is fine, often involving spatial scales of microns, and since these x-ray techniques are usually made as contact x-ray films, allowing virtually no magnification during the x-ray image acquisition, the resulting films may have to be scanned manually under high magnification using a conventional optical microscope. While a large portion of the fiber irregularities can be detected manually, this approach is manpower intensive and therefore expensive, slow, and tedious, resulting in errors even by experienced personnel. Also, a break in a fiber can often occur at the boundary of another superposed fiber, making visual detection virtually impossible. Moreover, since current x-ray techniques utilize a film detector, the images displayed cannot be easily digitized, and thus cannot be studied with current computer based image-processing techniques. Finally, non-film detectors, which may allow digitization but which have lower spatial resolution than film detectors, will not provide sufficient data to the computer because of the low spatial resolution of the recorded images.
Two advanced image x-ray imaging techniques, computed tomography and traditional high resolution digital laminography, also present serious limitations for imaging composites. Computed tomography (CT) returns cross-sectional images of the test specimen. Because of limitations in extended data acquisition time and in the number of picture elements in the reconstructions, CT does not present a practical approach for test specimens with large dimensions and fine spatial resolution requirements which require a large set of picture elements for reconstruction.
High resolution digital laminography serves to separate the planes of the plies, and to give good information as to the structure within the plies, all consistent with a spatial scale of microns. Laminography also allows one to image any selected internal plane independent of the material above or below it, removing confusing material above or below the surface of interest so that only that surface remains, thereby allowing an arbitrary surface in the test specimen to be reconstructed from multiple digitized images of the specimen.
Traditional laminography techniques also have serious shortcomings. As it is usually practiced, high resolution digital laminography utilizes a detector which is a high-resolution, x-ray sensitive, linear array which can only detect one dimensional images, (i.e., lines) at any one instant. In traditional laminography, a test specimen is moved through a plane PP defined by a source and two end points of the linear detector. Scanning an area of the test specimen requires multiple passes of the test specimen through plane PP, a pass being defined as a single sweep of the test specimen through plane PP. Each pass generates a single area image at the test specimen. On each successive pass, the test specimen is positioned in a different preselected orientation with respect to the source and the detector. To reconstruct any small region within the test specimen, absorption measurements from all views must be referenced/correlated to an accuracy consistent with about 1/3 of the spatial resolution of the final image. Such referencing is required to determine which imaged volume elements ("voxels") of the small region correspond to which datum points along the pass. Referencing all points along the pass (i.e., global referencing). requires precise knowledge of the relative positions of the source, test specimen and detector at each point along the pass (i.e., the procedure requires global mechanical registration). In addition traditional laminography scans the test specimen multiple times, the source being at a different orientation relative to the specimen for each pass; each additional pass thus provides a different single view of each voxel along the pass. Since data from all these different passes of the same voxels must also be correlated in order to reconstruct the region, mechanical registration of each line must be maintained between the multiple passes; and since the relevant data are acquired at different times over the entire data acquisition sequence, while the small region in question is scanned multiple times, mechanical registration is required over a spatial scale equal to the entire linear displacement of the test specimen, a distance that could be many feet in length, as the test specimen is performing complicated motions relative to the source/detector.
Achieving high mechanical registration over long, and sometime complicated passes is difficult because (a) precise position measurements are difficult and expensive to achieve and (b) both the relative position and absolute position of parts over long distances and over extended time periods are affected by multiple environmental, mechanical and other conditions which may result in continuously varying (systematic) or randomly varying errors. The sum of these errors, which is generally unpredictable, is referred to as a cumulative error. Since it is difficult to know with precision the position of the source, test specimen and detector at each point along each pass, using conventional laminography, such points can be accurately reconstructed at high resolution only with great difficulty.
In sum, one drawback of traditional laminography is the inherent need for multi-pass scanning which results in a slow, time consuming process. Another drawback is the requirement for high mechanical registration over multiple, frequently spatially-lengthy passes. Because of such drawbacks, it is difficult and time consuming to achieve high resolution images of large test specimens by traditional laminography. Moreover, traditional laminography is also not readily adapted for imaging curved test specimens.
Laminographic systems used in the medical imaging field also have shortcomings. Such systems image planes in the body along the longitudinal axis, are large and execute their motions in complex Lissajous figures. The images are recorded on film under the patient, such that only one plane is imaged for each scan. Further, there is only a 2 line pair/millimeter (lp/mm) or 0.01 in. resolution limit on most such systems, because of the severe mechanical accuracy requirements. The accumulated mechanical uncertainty over the pass must be less than approximately 1/3 of this number--approximately 0.003 in.--which is a difficult and expensive mechanical constraint to meet.
Another laminographic approach, also having drawbacks, is described by Bakes et al. in U.S. Pat. No. 5,081,656 and involves rotationally steering an electrically scanned x-ray beam across a mechanically stationary test specimen and x-ray detector. The digitally processed data can then be used to reconstruct any surface within the body of the test specimen. The limitations of this technique result from the requirement that the diameter of the circle traced out by the x-ray focal spot within the beam source housing be relatively small. Tracing such a small circle may not yield sufficient angular separation of views on thicker test specimens or provide sufficiently large fields of view in larger specimens. Thus, this approach is limited to relatively small and thin test specimens, such as circuit boards. Such prior art inspection technologies are therefore of limited utility in detecting breaks, debonding/delaminations, impact damage, and other flaws/anomalies in large complex composites.
Because current inspection methods are inadequate, in order to maintain the requisite margin of error/safety, particularly in critical applications, composite structures are overdesigned and manufactured, often with two to four times the fiber layer mass that would be required in the absence of undetected anomalies. This significantly reduces the weight advantages achieved using composites and greatly increases material costs. Similar problems exist for inspecting other multilayer, multidimensional structures.
A need therefore exists for a robust, efficient inspection technique for multilayer, multidimensional structures such as complex composites, which can isolate and image individual layers of a multilayer structure irrespective of thickness or layer geometry with minimal mechanical registration requirements.